Linux System Programming阅读笔记之 read(....)

关于read(...)返回值的正确判断：p30
File I/O 的 read(...)函数用法：
有问题的代码，只判断返回值为-1的情况。
unsigned long word;
ssize_t nr;
/* read a couple bytes into 'word' from 'fd' */
nr = read (fd, &word, sizeof (unsigned long));
if (nr == -1)
/* error */
Indeed, a call to read( ) can result in many possibilities:
• The call returns a value equal to len. All len read bytes are stored in buf. The
results are as intended.
• The call returns a value less than len, but greater than zero. The read bytes are
stored in buf. This can occur because a signal interrupted the read midway, an
error occurred in the middle of the read, more than zero, but less than len bytes’
worth of data was available, or EOF was reached before len bytes were read.
Reissuing the read (with correspondingly updated buf and len values) will read the
remaining bytes into the rest of the buffer, or indicate the cause of the problem.
• The call returns 0. This indicates EOF. There is nothing to read.
• The call blocks because no data is currently available. This won’t happen in nonblocking
mode.
• The call returns -1, and errno is set to EINTR. This indicates that a signal was
received before any bytes were read. The call can be reissued.
• The call returns -1, and errno is set to EAGAIN. This indicates that the read would
block because no data is currently available, and that the request should be reissued
later. This happens only in nonblocking mode.
• The call returns -1, and errno is set to a value other than EINTR or EAGAIN. This
indicates a more serious error.
正确做法：
ssize_t ret;
while (len != 0 && (ret = read (fd, buf, len)) != 0) {
  if (ret == -1) {
    if (errno == EINTR)
    continue;
    perror ("read");
    break;
  }
  len -= ret;
  buf += ret;
}